Oh I see. My work truck 2007 dodge quad cab has 160000 miles on it
and trans fluid still looks good. Runs great and has never been
changed. That being said it will probably fall to pieces next week. Lol.
Now mine has been run hard and put away wet way to many times and I
know its about time for me to do a trans filter and fluid change. But with
yours I would not worry about it till you start having issues. Now that
does not mean to drive it till it quits. If you start to notice it shifting a little
different or little noises that you haven't heard before then follow thru
with servicing it.

Your mechanic was telling you this based on GMs
maintenance schedule. GM says to change your
transmission fluid and transmission filter every 100,000
miles. BUT these transmissions, to be honest, are
troublesome and prone to failure. In this case, leave it
alone. If it works, don't fix it!! HTH. -Jim